The present invention relates to a device for selecting the overall cubic capacity of a hydraulic circuit comprising a first motor set comprising at least one first hydraulic motor for driving first vehicle-moving means for moving a vehicle, and a second motor set comprising at least one second hydraulic motor for driving second vehicle-moving means for moving the vehicle, said first and second vehicle-moving means being situated one behind the other in a direction in which the vehicle moves, the circuit further comprising first and second main ducts for the first motor set, and third and fourth main ducts for the second motor set, the first and third main ducts and the second and fourth main ducts being suitable for being connected in respective pairs to respective ones of two orifices of a pump for feeding and discharging the motor sets in parallel, the device comprising a selector suitable for taking up a first stable position corresponding to a first configuration of the selector device, in which configuration the third and fourth main ducts are connected to respective ones of the orifices of the pump, and said third and fourth ducts are isolated from each other so that the second motor set is active, and for taking up a second stable position corresponding to a second configuration of the selector device, in which configuration one of the first and second main ducts is connected to the third and fourth main ducts, so that the second motor set is inactive, the selector being such that, while the selector is moving between said stable positions, a temporary situation exists in which the first, second, third, and fourth ducts are interconnected.
A device of this type is known from Document U.S. Pat. No. 5,199,525.
The circuit to which the invention applies is, in particular the drive circuit of a vehicle having two driven axles to which the first and second motor sets are respectively coupled.
Each of the motor sets can have a single cubic capacity. In which case, when both motor sets are fed in parallel, the maximum overall cubic capacity of the hydraulic circuit is the sum of the cubic capacities of each of the two motor sets. When, with the selector device being in its second configuration, the second motor set is inactive, the overall cubic capacity of the circuit corresponds to the cubic capacity of the first motor set which remains active.
Naturally, the invention also applies to a configuration in which at least one of the motor sets has variable cubic capacity. In particular, the first motor set can have two distinct operating cubic capacities, while the second moor set has a single cubic capacity. The circuit can then have three distinct overall cubic capacities:                a maximum overall cubic capacity when the first motor set is in large cubic capacity mode and when the selector device for selecting the overall cubic capacity is in its first configuration so that both motor sets are active;        an intermediate overall cubic capacity when the first motor set is in large cubic capacity mode and when the device is in its second configuration, so the second motor set is inactive; and        a minimum overall cubic capacity when the first motor set is in small cubic capacity mode and the second motor set is still inactive.        
Naturally, different combinations can be imagined, e.g. another, intermediate cubic capacity in which the first and second motor sets are active, but in which the first motor set is in small cubic capacity mode.
The vehicle-moving means can be of any known types, such as wheels, tracks, a ball, etc.
The term “motor set” is used to mean a set of at least one motor coupled to vehicle-moving means. For example, if the vehicle-moving means are wheels, and if each axle has two wheels, each motor set can either comprise a single motor driving both of the wheels on the same axle, or else each set can comprise two motors coupled to respective ones of the two wheels and fed in parallel. Each motor can be of any type known per se, e.g. it can be constituted by a motor having a single cubic capacity and having two orifices serving respectively as a feed orifice and as a discharge orifice, or else by a motor having two distinct cubic capacities and made up of two sub-motors, each of which has a feed orifice and a discharge orifice, or indeed by a motor having two distinct cubic capacities and made up of two sub-motors having a common first orifice for feed or for discharge, and each having a distinct second orifice for feed or for discharge, the distinct orifices being suitable for being connected respectively to an orifice of the pump and to an orifice of a receiver connected to the same circuit. Such motors can be said to be “three-orifice motors”, and they are described in EP 0 547 947.
The invention applies to drive circuits for driving vehicles either having a preferred direction or not having a preferred direction. For a vehicle having a preferred direction (forwards), provision is made for that one of the first and second main ducts with which the third and fourth main ducts are put into communication in order to make the second motor set inactive to be the main duct that serves as the discharge duct for the first motor set in said preferred operating direction of the vehicle.
For such a vehicle, the non-preferred direction, i.e. reverse, corresponds to movement at low speed, during which movement the selector device for selecting the overall cubic capacity of the circuit is not normally used.
For driving a vehicle with no preferred operating direction, the circuit can be equipped with a valve making it possible, for deactivating the second motor set, to select that one of the first and second main ducts that is at the lower pressure, and to connect it to the third and fourth main ducts.
The temporary situation in which the first, second, third, and fourth main ducts are interconnected while the selector is going between said two stable positions serves to prevent the selector from being blocked and to prevent the feed to the second motor set from being blocked. In this provisional situation, all of the main ducts communicate with one another, and the vehicle freewheels.
Since both motor sets are fed by the same pump, going from one overall cubic capacity to the other causes a jolt in the drive of the vehicle. For any given flow-rate of the pump, the total torque delivered by the motor sets is higher but their speed is lower in the large overall cubic capacity situation than in the small overall cubic capacity situation. Therefore, when the selector device goes from its first configuration to its second configuration, the circuit goes into small overall cubic capacity mode, and an acceleration jolt is generated. Conversely, when the selector device goes from its second configuration to its first configuration, a braking jolt is generated. The braking jolt can be avoided or attenuated by the operator if, at the same time as the operator causes the device to go from its second configuration to its first configuration, said operator rapidly increases the flow rate of fluid delivered by the pump. Causing such an increase to take place is difficult and can be automated only by using a relatively costly device.
While the selector device for selecting the overall cubic capacity is going into its second configuration, the second motor set is deactivated, so that the drive means coupled to said second set naturally cease to be driving. Thus, the vehicle is then driven only by the drive means coupled to the first motor set which is active, but whose speed increases suddenly. On certain types of ground, such a sudden increase in speed can give rise to loss of grip, which is detrimental to the handling of the vehicle. Similarly, while going from the second configuration to the first configuration, the vehicle-moving means are suddenly driven at low speed. This jolt can also give rise to loss of grip and, since the drive means of the two axles are synchronized only via the ground, such loss of grip can give rise to loss of synchronization, which is also detrimental to the handling of the vehicle and to controlling its path.
As indicated above, the temporary situation in which the first, second, third, and fourth main ducts are interconnected, serves to prevent the device from being blocked. Up until now, for circuits in which the two motor sets drive distinct axles, ways have always been sought of making this provisional situation as short as possible.
EP 0 294 662 proposes to deactivate one of the motor sets of a circuit by connecting its main ducts to the reservoir, thereby making it possible to cause that motor set to freewheel by retracting the pistons into the cylinders. A temporary situation, in which the various main ducts communicate with one another, is generated by means of a selector whose movement is controlled.
The invention concerns a different context, in which the deactivated motor set is put into freewheel mode not by retracting the pistons into the cylinders by rather by being bypassed, its main ducts being connected together and to one of the main ducts of the other motor set. In addition, EP 0 294 662 does not make it possible to avoid jolts at the end of the movement of the slide because, in the intermediate situation, the motor set that was deactivated or that is to be deactivated does not deliver any torque.
An object of the invention is to remedy the phenomenon of jolting that is observed when changing the overall cubic capacity of a circuit of the type indicated in the introduction, or at least to attenuate that phenomenon.
This object is achieved by the fact that the selector device of the invention further comprises means serving, at least while the selector is moving in a first direction between its first and second stable positions, to sustain said temporary situation for a period of time in an intermediate stage during which an interconnection between the third and fourth main ducts and an interconnection between at least one of the first and second main ducts and at least one of the third and fourth main ducts are constricted.
Thus, with the invention, the temporary situation is maintained so as to enable fluid flow rate exchange to take place between the main ducts and thus, so as to avoid a sudden variation in said flow rate. The motors are not totally deactivated in this temporary situation because the above-mentioned interconnections are constricted in this situation, so that the main ducts do not communicate freely with one another.
By means of the constriction in the interconnection between the third and fourth main ducts in the intermediate stage, the second motor set then delivers limited torque that depends on the head loss due to said constriction. As a result, while the selector is finishing its movement, the increase or the decrease in the total torque delivered by the motor sets is much smaller than in the above-mentioned prior art, and is thus not sudden, thereby making it possible to avoid jolting.
In particular, the temporary situation is maintained while the selector is moving from its second position to its first position, i.e. during a movement of the selector that leads to an increase in the overall cubic capacity of the circuit. For example, if the first and second motor sets are serving to drive respectively the wheels of the first axle and the wheels of a second axle of a 5 (metric) tonne (T) vehicle moving at a speed of about 10 kilometers per hour (km/h) at the time of the cubic capacity change, and if the torques delivered by the motors respectively in small overall cubic capacity mode and in large overall cubic capacity mode are respectively 1500 newton-meters (N.m) and 3000 N.m, the period of time for which the temporary situation is sustained can be of the order of in the range 3/10ths of a second to 5/10ths of a second, or even of the order of 1 second. The same applies for two motor sets each driving a respective one of the vehicle-moving member sets of a 24 T vehicle (i.e. 12 T per motor set) at a speed of the order of 3 km/h at the time of the cubic capacity change, the torques delivered by the motor sets respectively in small cubic capacity mode and in large cubic capacity mode being respectively 2000 N.m and 4000 N.m. For sustaining the temporary situation, the movement of the selector can be slowed down or braked, or even substantially stopped for the necessary period of time.
It is possible to choose to implement the invention in such a manner as to limit the jolting in only one change direction in which the valve goes from one position to the other, e.g. on going from the small cubic capacity to the large cubic capacity.
It is also possible to choose to limit the jolting in both change directions, by making provision for an intermediate stage as indicated above to exist in both directions. As explained below, it is possible to choose to make provision so that, in the intermediate stage, the cross-section of at least one of the passages between the first and third ports and between the second and third ports is reduced, while the cross-section of the other passage makes it possible for fluid to flow substantially freely. It is also possible to choose to make provision for the cross-sections of the two passages to be constricted or calibrated while being different from each other.
Advantageously, during the intermediate stage, the interconnection between the third and fourth main ducts and the interconnection between at least one of the first and second main ducts and at least one of the third and fourth main ducts have constricted cross-sections that are different.
For example, the interconnection between the third and fourth main ducts is less constricted than the interconnection between at least one of the first and second ducts and at least one of the third and fourth main ducts.
Advantageously, the selector is a valve having at least three ports, comprising first, second, and third ports connected respectively to the first, to the second, and to the third main ducts, and the device further comprises means for interconnecting the second and the fourth main duct, the first and second main ducts being respectively connected to the first and to the second orifices of the pump.
In which case, it is possible to choose to sustain the intermediate stage while going from the second position to the first position, i.e. on going from the small overall cubic capacity to the large overall cubic capacity of the circuit, while making provision so that, in the intermediate stage, the passage between the third port and the second port forms a constriction while the passage between the first port and the third port is substantially non-constricted or, at least, is less constricted than the passage between the third port and the second port.
In which case, if it is considered that the vehicle is traveling forwards, and that the main duct is connected to the fluid feed, the first and third ports being put into communication with each other causes the cubic capacity that is to be fed to be increased, so that the pressure tends to decrease in the feed first main duct, with this decrease being compensated by boosting. At the same time, since the second and the third ports are put partially into communication with each other, via the above-mentioned constriction, the increase in the pressure in the second main duct that serves as the discharge duct is limited. The constriction causes head loss. This makes it possible to manage the appearance of backpressure in said duct and therefore to control the braking of the vehicle.
Since the movement of the vehicle is slowed down in the intermediate stage, the second port and the third port continue to be put into constricted communication with each other for the time necessary for the kinetic energy of the vehicle to be consumed by the braking effect of the motors.
Advantageously, the valve has a fourth port connected to the fourth main duct and, when the selector is in the first stable position, the first and second ports and the second and fourth ports are interconnected in respective pairs, and, when the selector is in the second stable position, the third and fourth ports are interconnected and are connected to one of the first and second ports while being isolated from the other of the first and second ports, whereas, in the temporary situation, the first and second ports are connected via a first passage having a first constriction, the second and fourth ports are interconnected via a second passage having a second constriction, and an additional passage having an additional constriction exits between the first and second ports or between the third and fourth ports.
When the selector is a valve of this type, it has analogous connections to the first and second main ducts and to the second and fourth main ducts.
Advantageously, the selector device further comprises means for causing the selector to move between said stable positions, during which movement said temporary situation exists, which control means are suitable for causing a fast movement first step to take place between the initial stable position of the selector and a position corresponding to the beginning of the intermediate stage, and for causing a braked movement step to take place during which the intermediate stage takes place.
Advantageously, the control means are also suitable for causing another fast movement step to take place towards the other stable position, at the end of the intermediate stage.
Thus, the movement of the selector is slowed down during the intermediate stage only, without the other stages in the movement of the selector being slowed down, thereby making it possible to prevent the time taken to go from one position to the other from being too long.
For example, the speed of movement of the selector during the intermediate stage is at the most equal to one third of its speed of movement during the fast movement first step. For example, the slowing down is such that the length of time of the intermediate stage is at least equal to two-thirds of the total time taken by the selector to move between its two end positions.
Preferably, the selector is a valve comprising a slide mounted to move in a bore, the slide being suitable for taking up two opposite positions corresponding to respective ones of the first and second stable positions of the selector, and, while it is moving between these two opposite positions, being suitable for taking up an intermediate position corresponding to said temporary situation.
The control means for causing the selector to move can be hydraulic, electronic, or indeed a combination of hydraulic means and of electronic means. When they are present, the electronic means advantageously comprise a servomechanism. These control means can be of the type described in EP 1 058 002 or WO 01/61186.
Thus, the selector device advantageously further comprises means for causing the selector to move between said stable positions, during which movement said temporary situation exists, which means are suitable for servo-controlling a variable that is representative of operation of the vehicle to a set point by causing the selector to move in controlled manner between said two positions.
The representative variable is preferably determined on the basis of at least one of the parameters constituted by the pressure in at least one of the main ducts, the flow rate in at least one of the main ducts, the output torque of at least one of the motors, the number of revolutions at the outlet of at least one of the motors, the speed of the vehicle, position relative to a target, distance relative to a target, the acceleration or the deceleration of the vehicle, the acceleration or the deceleration of at least one of the motors, an operating parameter of the pump, the power being consumed, an operating parameter of an auxiliary motor driving said pump, and an operating parameter of a tool fed hydraulically from the circuit.
Advantageously, the control means for causing the selector to move further comprise electronic control means, and said device further comprises means for measuring the value of at least one critical parameter, for computing the value of the representative variable, and for causing the selector to move from one to the other of its stable positions as a function of the value of the computed representative variable.